The present invention relates to the determination of input signal thresholds for devices and more particularly to a method of using calibration instrumentation for determining the operating and the reset points for a limit value device.
This invention relates to a method of determining an operating and a reset point for a limit value device, the device having an input and an output, the output having two (2) states (operating and reset) and the input having threshold points associated therewith for triggering output state changes via an input signal presented at the input of the device. Such devices are sometimes referred to as limit value devices. The exact input values of the operating and reset characteristics and values are not known with certainty prior to using the method with calibration instrumentation to determine the operating and reset points more precisely. The method utilizes variable input signals which are supplied to the limit value device, when the limit value device changes state (operate-to-reset or reset-to-operate) both the value of the input signal and the direction of the change of state are detected.
A procedure as described above is known from Finnish Published Specification 81657, in which the limit value device is a pressure switch. The calibration instrumentation employing the principle of defining an operating and a reset point for a limit value device, known also from this publication, is illustrated by means of a block diagram, shown in an enclosed FIG. 1 and referred to in the following. A limit value device 1 is a device the output of which consists of a switch function or a logic ON/OFF function. Input 2 is some physical quantity, such as voltage, current, resistance, frequency, pressure or temperature. When the input exceeds a certain limit, output 3 changes its state, and when the input falls below some other limit, the output 3 resets to its original state. Between the operating point and the reset point, there is usually hysteresis. With some devices, the recurrence (retriggering) of the operating point and the reset point can be relatively hard to determine.
The purpose of checking a limit value device is to find out at which point the device operates and at which point it resets. If the repeatability of the operate and reset states of the device is bad or hard to predict, it is possibly desired that the testing be repeated several times. If the device does not operate satisfactorily, it may be required that its operating and reset points be adjusted or tuned and the test is repeated after that.